PlankwayHow Plankway puzzles are made
Every Plankway puzzle is generated by a program, but not at random. Each one has to pass the same tests a careful puzzle editor would apply: exactly one solution, reachable without guessing, and of the right difficulty for its day.
1. Build a network
The generator starts with one island on an empty grid and grows a network from it, bridge by bridge. Each new bridge runs straight from an existing island to a new one, without crossing anything. Some bridges become double bridges, and sometimes an extra connection between two existing islands is added, which creates loops and makes the puzzle less linear.
When the network is big enough, the numbers are simply counted: each island gets the number of bridges that end on it. Then the bridges are removed. What is left is the puzzle.
2. Add reefs
Next, reefs are placed in the water. A reef may never sit on a bridge of the planned solution, but it may block other connections, which is exactly what makes them interesting: they remove options and change how islands see each other.
3. Prove there is one solution
A counted network is not automatically a good puzzle: there may be other ways to satisfy the same numbers. So the generator runs a complete search that looks for a second solution. If it finds one, the puzzle is thrown away.
Then a second solver tries the puzzle the way a person would, using only the techniques in this guide, from full island up to try and see. If it gets stuck without guessing, the puzzle is thrown away too.
4. Rate the difficulty
Each technique has a weight, from 1 for the counting techniques to 6 for try and see. The difficulty of a puzzle is the weight of the hardest technique it needs, with the number of steps as a tie-breaker. Each day of the week has its own target (see the weekly difficulty); the generator keeps trying, up to 200 attempts, until it finds a puzzle that fits.
The same puzzle for everyone
The generator is deterministic: given the same seed, it always builds the same puzzle. The daily puzzle uses the puzzle number as its seed, so the server and every player’s phone arrive at exactly the same board. That is also why Plankway works offline: your device can build today’s puzzle by itself if needed.
Because the solver records every step it takes, we can also show how each past daily is solved. That is what the puzzle archive is: the solver’s own reasoning, written out for every puzzle.